1) Katie is especially vulnerable to this sort of damage, yes.

2) Generally, it's something that could work! He'd be able to track whichever shade he's put it on, but not all of them at once. It could last for up to one night and reset with the dawn. This particular prompt will have NPC interactions, and whether or not Garner is able to get it on her will be dependent on what he does in a given thread.